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A 24-year-old PhD student has been identified as the masked gunman who opened fire at a movie theater in Colorado early Friday, killing 12 people and injuring 59 others.

The shooting occurred in the Denver suburb of Aurora at the 16-theater Century complex, which was packed with hundreds of people, including many children, for the midnight opening of the Batman movie "The Dark Knight Rises."

James Holmes, originally from Tennessee, was detained shortly after the shooting spree in a parking lot behind the cinema complex.

One of the moviegoers, James Wilburn, told the Denver Post he saw a man dressed in black and wearing a mask enter the theater through an emergency door alongside the screen.

The man tossed a tear gas canister, raised a shotgun and began shooting at people in the audience, Wilburn said.

After exhausting the ammunition in the shotgun, the shooter calmly dropped the weapon, shouldered a rifle and resumed firing, the witness recounted.

Wilburn said he heard around 30 shots as he and his friends hid behind the backs of seats.

Aurora Police Chief Dan Oates told local television that officers apprehended the suspect in possession of a shotgun, a rifle, a pistol and a bullet-proof vest.

The man later identified as Holmes also told the officers he had explosives at his apartment.

Police subsequently found a plethora of explosive devices and tripwires at Holmes' apartment.

"The pictures we have from inside the apartment are pretty disturbing considering how elaborate the apartment is booby trapped," police said outside of the apartment complex today.

They said Holmes' unit contained enough "flammable and explosive" material to blow up the entire complex and five nearby buildings, which were ordered evacuated.

President Barack Obama, who was on a political visit to Florida at the time of the shooting, canceled all campaign events in the face of the "national tragedy."

His Republican opponent, Mitt Romney, also suspended campaigning.

"Our hearts break for the victims and their families," Romney said. EFE